Frequency synthesizers have a wide range of applications, including wide-band communications systems and information-processing systems. One of the elements of a frequency synthesizer is a voltage-controlled oscillator (VCO). A commonly used VCO is an inductive-capacitive (LC) VCO. In order to meet the requirements of modern wide-band frequency synthesizers, it has traditionally been necessary to utilize VCOs with large gains (Hz/V). However, chip integration has now increased to the point that VCOs with large gains often suffer from noise coupled through the voltage supply and substrate. In an effort to avoid this problem, prior art systems have begun to utilized VCOs with lower gains and with switched-capacitor banks that can be switched on and off to cover a wide range of frequencies.
Due to variations in the operating frequency, voltage, temperature and processing of a VCO, it is necessary to calibrate the switched-capacitor bank for a given required operating frequency. Prior art systems for calibrating switched-capacitor banks generally compare the reference-clock signal and the divided-clock signal of the frequency synthesizer by counting the duration of these signals with reference to the reference-clock signal. This comparison indicates which clock signal is running fastest. In order to perform an accurate comparison, the two clock signals must be compared over a large number of periods. As a result, prior-art calibration systems require a relatively long time to calibrate a frequency synthesizer.